1. Field of the Invention
The present invention relates to an inductor circuit, and more particularly to a parallel-structured inductor circuit which has an improved quality factor Q and is optimized at operating frequencies due to variable inductance.
2. Description of the Related Art
In general, center-frequency-variable radio frequency (RF) circuits are needed in wide-band communication systems such as televisions, ultra wide band, and the like, and in multi-band communication systems such as mobile phones, personal communication service (PCS), wide-band code-division multiple access (WCDMA), and the like, which can support multiple bands.
The frequency-tunable RF bandpass filter is one of such RF circuits capable of reducing power consumption with much less strict requirements of the linear characteristics of RF circuits and the phase noise of local oscillators since the filter enables a user to select his or her intended channels in RF bands and removes interference signals outside the intended bands as well as interfere signals from neighboring channels.
In order to change operating frequencies, such conventional frequency-tunable RF bandpass filters, amplifiers, mixers, voltage-controlled oscillators (VCOs), or the like, use voltage-controlled varactors controlled with an analog tuning signal or switched capacitors with capacitors connected in parallel with each other for switching. However, there exists a disadvantage to such a method of varying operating frequencies by changing capacitance due to a narrow variable range of operating frequencies, poor noise characteristics, and degraded oscillation performance. In particular, the VCOs require a scheme of varying the center frequency by using inductors in order to optimize the phase noise.
Thus, there has been a scheme using inductors for varying operating frequencies by changing inductance. FIG. 1 is a circuit diagram for showing a VCO having a modified inductor structure developed by the University of Florida in 2001. As shown in FIG. 1, a two inductors L3 and L4 are connected in series, and the inductor L3 is connected in parallel with a switch M4. The inductors L3 and L4 are small in size, have inductance optimized at high operating frequencies when the switch M4 is turned off and have inductance optimized at low operating frequencies when the switch M4 is turned on. However, such a VCO circuit has a disadvantage of difficulties in optimization at low frequencies since the capacities of the inductors L3 and L4 are small and the VCO circuit is optimized at high operating frequencies if the two inductors L3 and L4 are in operation. Further, the VCO circuit has a disadvantage of degrading its quality factor Q indicating the inductor performance since resistance of the switch M4 has influence on the inductor M4 connected in parallel therewith when the switch M4 is turned on.
Further, FIG. 2 is a circuit diagram for showing an inductor structure disclosed in U.S. Pat. No. 6,549,096 entitled “Switched inductor/varactor tuning circuit having a variable integrated inductor”. A pair of inductors L5 and L6 each forming a separate circuit is connected in parallel with each other and a switch is connected to the inductor L5 to which electric power is not supplied. If the switch is opened, the inductor L5 has inductance optimized at high frequencies. If the switch is closed (turned on), electric current is induced in the opposite direction to that of the inductor L6 by the inductor L6 connected to the switch. Thus, since eddy current is generated between the inductors L5 and L6, resistance increases and inductance decreases. Accordingly, the inductor structure shown in the prior art 2 has a disadvantage of considerably degrading the quality factor Q as the resistance increases.
Thus, an alternative scheme is needed to enable the optimization at operating frequencies and the quality factor (Q) improvement by providing a new inductor structure to vary inductance.